Native backfill material was desired as test results showed that native soil would adequately dissipate heat away from the electrical lines to allow the lines to carry the ampacity needed for the installation.
Working on a tight two-week schedule, the crew needed to dig a 10,000-foot-long ditch that was 36 inches wide and had a 54-inch-deep floor.
The Morse Group, an Illinois-based electrical contracting firm, was just about to start a Wisconsin installation of direct-burial high-voltage underground cables for six 262-ft. wind turbines, each with three 135-ft. blades, when it was informed that the engineers wanted to switch from hauling in fill to using native material.
Bruce Binger, a 20-year industry veteran and project superintendent for Morse, recalls, “The engineers learned from test results back just two days prior to the scheduled start of excavating that the native soil would adequately dissipate heat away from the electrical lines to allow them to carry the ampacity needed for the installation. So they wanted to use the native material to backfill as much as possible.”
While this would reduce the need to purchase and haul in outside fill material, the native soil contained a lot of rocks because this area had glacier remains. The material couldn’t go back into the ditch without separating the rocks out.
No Time to Waste
The development of the wind farm in Dane County for a private software company is located just northwest of Madison, and is the county’s first commercial wind farm. The company has already installed a geothermal system and solar panels in its facilities. The addition of the wind turbines to the company’s current sustainable efforts will help it provide around 85% of its own energy by 2014.
The project started in October, and the company wanted to complete the installation of the wind turbines by the end of the year. This tight timeframe meant that Binger needed to find a way to separate the good soil from the rock quickly to keep the project on schedule.
It was after lunch on a Wednesday when Binger learned his crew would have to change its backfilling plan, yet it still had to start digging by the weekend. To identify possible screening options and solutions, he did an Internet search and discovered the hydraulically-powered ALLU line of screener crusher processor attachments. The screener crusher processor is designed to separate soil from rock and other debris, and would fit on The Morse Group’s 24-metric-ton Volvo excavator.
The attachment was delivered by Friday and began onsite backfilling by Saturday at noon. The crew, working on a tight two-week schedule, needed to dig a 10,000-ft.-long ditch that was 36 in. wide and had a 54-in.-deep floor. It went from a main switch gear installation, from which the electrical lines originate, to each of the towers.
The three operators of the excavator and attachment required only minimal training on how to make material fall succinctly into the bucket’s drums for best production. It was then able to place 18 to 20 in. of cover immediately over the electrical cables. The first lift of processed fine soil did not require direct compaction. The crew was able to use the rocky spoils it placed off to the side for later upper-lift placement.
The project screening requirements were for 3/4-in. diameter and below and the screener crusher processor met that requirement. “You can really tell the difference when you walk on it,” says Binger. “If you were to just take a regular backhoe bucket and fill the ditch, and then take an ALLU processor and fill the ditch, there would be a lot less settling involved after with the area using the screener bucket. The fines just automatically pack in and lock together better.”
Material and Equipment Cost Savings
By using the attachment, The Morse Group avoided the need to haul in soil by the truckload and rent additional carriers such as wheel loaders to move the material, or alternatively placing flow-able fill that would be delivered by ready-mix trucks. In addition, the project was located in the middle of a farm field that was wet some days and would have made it more difficult to get this hauling equipment in and out.
Since the company could use its existing excavator, it required only the screener crusher processor be purchased. The excavator worked well in wet conditions and the bucket was still able to separate native material from the rock and debris, even when it received an inch of rain overnight. This all added up to a significant savings.
“It’s built heavy, and all of our operators have been very happy with its performance.” says Binger. “We needed something quick. The job that we were on was a very fast-paced project – things were evolving very rapidly – but the machine worked exactly like ALLU had described and exactly what it looked like in videos on the Internet.”
The Morse Group has been so impressed with the screener crusher attachment that it planned to rent another one, plus use the current one, for an upcoming project.